1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high powered pulse lasers, in shock processing of solid materials, and more particularly to methods and apparatus for determining if sufficient laser shock processing has occurred by determining vibrational frequency and mode shapes of workpieces, such as gas turbine engine blades.
2. Description of the Related Art.
Airfoil failures, i.e., cracking, separation, and distortion, result from a number of different causes, one being vibratory resonance. Gas turbine engine blades occasionally respond in their operating envelope by resonant mode vibration between their start, idle and maximum RPM speeds.
Laser shock processing has been used for processing metal parts as shown U.S. patent application Ser. No. 08/962,622, now U.S. Pat. No. 5,988,982, laser peening systems may change the vibrational frequencies of most workpieces, such as gas turbine engine blades.
According to the present invention a method for determining the vibration characteristics of a workpiece, particularly a gas turbine engine blade, is taught. Laser shock peening is applied to locations on the workpiece, particularly to areas of high bending strains between or near the nodal lines for a particular preselected vibration mode. The use of laser shock peening is used to modify the airfoil or the workpiece natural vibration frequencies and mode shapes to improve their operating characteristics.
The invention, in one form thereof, includes a method for determining that a production workpiece has been sufficiently laser shock peened, the method comprising the steps of: determining a natural frequency of a test workpiece before laser shock peening said test workpiece; determining the frequency shift of said natural frequency of said test workpiece after said test workpiece has been laser shock peened; comparing said frequency shift to a predetermined acceptable range of frequency shift for said test workpiece; processing production workpieces if said frequency.
The invention includes a method for determining that the laser shock peening processing conditions are within an acceptable range, the method comprising the steps of: determining a natural frequency of a test workpiece before laser shock peening said test workpiece; determining the frequency shift of said natural frequency of said test workpiece after said test workpiece has been laser shock peened; comparing said frequency shift to a predetermined acceptable range of frequency shift for said test workpiece; and processing production workpieces if said frequency shift is within said acceptable range of frequency shift.
In another form of the invention, a method is described for determining that a workpiece has been sufficiently laser shock processed, the method comprising the steps of: determining a natural frequency of a laser peened workpiece; and comparing said determined natural frequency to a predetermined natural frequency indicative of a sufficiently laser shock peened workpiece.
Another form of the invention includes an apparatus for determining that a workpiece has been sufficiently laser shock processed, the apparatus comprising:
a laser to laser shock process a workpiece;
a frequency measuring device to measure a natural frequency of the workpiece operatively associated with said laser;
comparing means for comparing said natural frequency to the acceptable condition of one of range of frequency and frequency shift for the workpiece, wherein if said frequency shift or frequency is not within the acceptable range, said means causes said laser to again laser shock process the workpiece until said natural frequency condition is satisfied.
An advantage of the present invention is that a natural frequency shift of the laser peened part may not be used to determine whether or not a sufficient laser peening activity has been accomplished on the part. Such a measurement of the frequency shift is a non-destructive evaluation technique suitable for a production processing environment of workpieces, such as gas turbine engine blades.
An advantage of the present invention is that it reduces vibratory amplitudes that may damage the workpiece, such as gas turbine engine blades, at particular frequencies. Such change in vibration frequency and mode shape may improve workpiece or blade lifetime, and for blades, the engine operating envelope.
Another advantage of the present invention is, the use of laser shock peening can be utilized to modify the airfoil and vibration frequencies and mode shapes in the design stage to improve the operating characteristics of the blade and engine. Additionally, the method may be used for an after-manufacturing solution to vibration problems encountered during engine operations or modification of existing blades to minimize a potential problem.
A further advantage of the invention is that by moving the resonance frequency of the workpiece by a fraction, even as small a range as two to five percent, high amplitude beating at a particular initial resonance frequency is eliminated, thereby reducing high fatigue stresses at the node of that vibration mode.